Stretch films that self-seal when portions are overlapped are known as "cling" films. These films are most often multilayer films and are used in applications where it's desirable to securely hold and/or wrap an article or group of articles, such as load palletization. For load palletization operations, the film is stretched tightly around the good or plurality of goods situated on a pallet, and self-sealed while the film is still in the stretched condition to create a secure, unitized package. Both unstretched or pre-stretched films are used for load palletization and retention of substantial cling properties and good tear properties while the film is stretch-wrapped are required to maintain the integrity of the unitized load.
There are several deficiencies associated with known cling films. For instance, many cling films are characterized as having substantial cling properties on both their outer ("obverse") layer and their inner ("reverse") layer. These two-sided cling films retain unsightly dirt and dust and undesirably adhere to adjacent goods and similarly wrapped packages when abutted against one another. Adhesion to adjacent goods and packages typically causes ripping, tearing, distortion and/or unraveling of the film during subsequent handling.
As disclosed in U.S. Pat. Nos. 5,175,049 and 3,025,167, cling films are often prepared by the addition of minor amounts of low molecular weight tackifiers. Common tackifiers include polybutylenes, mono- and diglycerides of fatty acids, amorphous polypropylenes, terpene resins and rosin esters. Tackifiers are non-film-forming, migratory materials that can cause die lip build-up during film fabrication and undesirable accumulations on equipment during wrapping operations. Die lip build-up and other accumulations on equipment require periodic stoppage of operations for cleaning and maintenance. Moreover, tackifiers often migrate to adjacent and opposite film layers and cause the aforementioned undesired adhesion and dirt retention problems commonly associated with two-sided cling films.
U.S. Pat. No. 5,093,188 teaches cling films comprising blends and/or multilayer constructions of chemically distinct polymers. An example of a cling film based on chemically distinct polymers is disposed in U.S. Pat. No. 5,172,343 where an ethylene acrylate cling layer is used opposite a polypropylene or polyethylene non-cling layer. However, the use of functional polymers such as ethylene acrylate polymers and ethylene vinyl acetate polymers can present rheology matching problems in coextrusions with linear polyethylene as well as thermal instability and incompatibility problems during recycling of edge trim and film scrap generated during the film fabrication step. While rheological, instability and incompatibility problems can be mitigated to some extent by the use of extrusion-grafted linear polyethylene polymers as the functional polymer layer, as opposed to using the more common high-pressure autoclave produced polymers, these problems are not completely eliminated by employing the extrusion-grafted linear polyethylene polymers.
The addition of tackifiers or the use of functional polymer layers are allegedly not required for some prior art films. For example, EPO 0 287 272 discloses cling films can be prepared from heterogeneously branched linear low density polyethylene (LLDPE) with relatively high levels of n-hexane extractables. Preferred LLDPE polymers have a density in the range of 0.905 grams per cubic centimeter (g/cc) to 0.940 g/cc. However, these films tend to show inconsistent cling properties as well as the accumulation and migration shortcomings generally associated with cling films prepared by the addition of tackifiers.
Stretch wrap films having only one outer layer with substantial cling properties are known as one-sided cling films. Ordinarily, the obverse layer is the "non-cling" layer of the one-sided cling film and it forms the exterior of a package or wrapped article.
One-sided cling films can be prepared by corona discharge treatment of monolayer and multilayer films. However, as indicated in U.S. Pat. No. 4,425,268, functional polymers and copolymers are still required to obtain the desired cling properties. As such, and since functional polymers generally do not exhibit sufficient strength properties for stretch wrapping operations, multilayer constructions involving high strength linear polyethylene layers are more frequently used.
Typical multilayer constructions comprise an A/B or A/B/C film structure where the A layer is the cling or reverse layer and the B layer (or the C layer in an A/B/C construction wherein the B layer is a core or structural layer) is the non-cling or obverse layer. However, as discussed above, multilayer cling films usually comprise polymers with high levels of n-hexane extractables, low molecular weight tackifiers, or functional ethylene polymers or grafted polymers, or combinations thereof. There are several disclosures that relate to such multilayer cling films, including U.S. Pat. Nos. 3,501,363 3,508,944, 3,817,821, 3,986,611, 4,022,646, 4,082,877, 4,303,710, 4,348,455, 4,399,180, 4,364,981, 4,379,197 4,418,114, 4,425,268, 4,436,788, 4,504,434, 4,518,654, 4,542,188, 4,588,650, 4,612,221, 4,671,987, 4,833,017, 4,923,750, 5,049,423, 5,066,526, 5,093,188, 5,114,763, 5,141,809, 5,175,049, 5,208,096, and 5,212,001, all of which are incorporated herein by reference. While multilayer films are most often used for stretch cling wrapping operations, the multilayer construction in itself does not resolve the aforementioned deficiencies. These deficiencies are largely due to polymer selection and their respective chemistries and, generally, are not due to how the materials are brought together or fabricated into film.
Multilayer films can be successfully produced by number of well known methods including coextrusions and hot nip and adhesive film laminations as described in the various references incorporated herein. Load palletization can also be successfully accomplished manually or by a number of automated methods including the methods and apparatus described in U.S. Pat Nos. 5,054,263, 5,020,300, 4,779,396, and 4,754,594. However, overall deficiencies of inconsistent cling properties, die lip build-up, accumulations on equipment, and recycle incompatibility continue to persist in the art of stretch cling films.